The long-term goal of this research is to understand the structural and functional changes in the hypothalamus that are responsible for the reversible suppression of ovarian function, such as occurs prior to puberty. It has been recognized for over 40 years that increases in response to estradiol (E2) negative feedback play a major role in producing these periods of infertility (e.g., the gonadostat theory of puberty), but the underlying changes in hypothalamo-pituitary function remain largely unknown. Using seasonal breeding as a model, we have identified a neural circuit (the vmPOA-RCh system) that plays a key role in alterations in E2 negative feedback in the ewe. This circuitry mediates E2 inhibition of gonadotropin-releasing hormone (GnRH) pulse frequency and is activated during periods of anovulation (anestrus) and inactivated during periods of fertility (breeding season). The first specific aim will better characterize this circuit by determining the phenotype of estrogen-responsive elements in the vmPOA and RCh and evaluating the role of ER in mediating the actions of E2. The second specific aim will test three alternate explanations for the activation of this system in anestrus: an increase in estrogen receptors, an increase in key neurotransmitter levels, or an increase in synaptic contacts within the circuit. The third specific aim will begin to determine the mechanisms by which external factors control the activity of this circuit. There is now evidence that another hypothalamic area, the premammillary region (PMR) controls changes in responsiveness to E2 negative feedback. The experiments in Specific Aim 3, we will examine the anatomical and functional connections between this area and the vmPOA-RCh system and test the hypothesis that it is critical for changes in response to E2 negative feedback. These questions will be addressed using a combination of anatomical, molecular and physiological approaches that include immunocytochemical analysis (at the light and electron microscopic levels), in situ hybridization, tract-tracing, local administration of drugs and hormones to different hypothalamic areas, and the production of small hypothalamic lesions. The results of these studies will provide fundamental information on the control of reproductive function and on neural plasticity in the adult brain that may be relevant to both physiological and pathological suppression of ovarian function.